A Robot that Flies like a Bird

September 26, 2011 at 2:08 am

For centuries, man has been possessed by the desire to fly. Naturally, nearly every inventor drew inspiration from the flight of birds. Nearly all of the earliest designs for flying machines attempt to replicate a bird’s flapping wings. Unfortunately, replicating a bird’s flight is exceedingly more difficult than one might anticipate. In fact, the motion is so sophisticated that no one has been able to construct a prototype that mimicks the complex motion until recently.

In July 2011, over 100 years after the Wright Brothers achieved manned flight, Markus Fisher introduced the world to his “robot that flies like a bird.” To fully appreciate his presentation, it is important to understand the primary differences between fixed winged flight the type of flight he has succeeded in replicating.

The primary difference between the two styles of flying is that fixed winged flight (like that of an airplane) segregates lift and propulsion into two separate actions while flapping wings combine them into one motion. As we already studied in class, a fixed wing generates lift due to the forces of the air on the underside of the wing. When a wing is angled (i.e., is positioned with an angle of attack) the leading edge of the wing pushes the air forward and down. Due to the greater amount of force placed on the bottom of the wing, the air forces it to rise up as expected based on Newton’s third law of motion. The force of the wing being pushed up causes the air on the top of the wing to increase in speed as it passes over. As the air molecules begin to move in the same direction (over the top of the wing) they no longer exert as much force down on the wing. In other words, they create a low pressure zone which enables the wing to lift upwards.

While the explanation for fixed winged flight seems complex, it is surprisingly simple in comparison to the method birds use to fly. Birds combine the already difficult concept of lift with propulsion. They do the two simultaneously. Biological wings can change shape and velocity as they flap. They slow down and stop at the end of downstrokes and upstrokes then accelerate into the next halfstroke. The wing base is always moving slower than the wing tip meaning that velocity increases from the base of the wing to the tip.

In addition, birds have the option of either using continuous-vortex gait (fast flight) or vortex-ring gait (slow flight.) Continuous-vortex gait produces lift during both the upward and the downward strokes of the wing. The downstroke produces a positive upward force and forward thrust while the upstroke produces a positive upward force and rearward thrust. In vortex-ring gait, the upward flap is passive and produces no thrust or lift.

As we take the time to study flapping wings, it becomes apparent that the motions birds use to fly is incredibly more complicated that what we have used in the past. It is also ironic, that it was the first form of flight modeled, and yet continuos to be the most difficult to replicate. With all the information in mind about how birds use their flapping wings to fly, perhaps we can better appreciate the sophistication and brilliance behind Markus Fisher’s robot.

One Comment

This is an excellent post, Charlie. You make a strong point, and one that is perhaps surprising to someone thinking about the physics of flight for the first time. Imitating birds was just too hard to be the solution for human flight!

A minor correction to your explanation of lift generation: the forces of the air on the wing are felt on both sides (not just the underside). The air flow is in fact upwards at the leading edge, and downwards at the trailing edge, and the resulting pressure distribution, as you correctly state, has lower values at the topside, resulting in a force upwards.